Media filters are well known in the art for use in removing contaminants from fluids, such as the removal of suspended solids from water. Media filters typically comprise granular filtration media, such as sand, anthracite, gravel or the like, arranged to form a filter bed within a filter vessel, wherein the fluid to be treated flows through the filter bed such that suspended solids become trapped in the interstices formed between individual granules of media.
Many filtration applications may also require the removal of a selected liquid component from a mixture or emulsion, such as the removal of oil from water. In such cases a filtration media may be selected which has an affinity for the liquid component to be removed. For example, the use of nutshells, such as walnut or pecan nutshells can remove oil from water, wherein the oil is adsorbed on to the surface of the nutshells. Nutshell filtration media is used in the oil and gas industry, for example for removing oil and other contaminants from water produced from a subterranean formation. Following a nutshell filtration stage, and other treatment stages if required, the filtered water may be disposed of or reused, for example by reinjection back in to the formation.
Over time the filtration media will become choked or clogged, for example by significant retention of suspended particles and/or coating with a liquid contaminant, resulting in a reduction in the efficiency of the filter. To address this, the filtration media is regularly cleaned, for example by backwashing, media scrubbing or the like.
Backwashing normally requires a volume of water to be flushed across the media in a reverse direction to remove filtered particulate matter. However, backwashing alone may not adequately remove all contaminants, such as oil, adhered to the media. In such cases it is known to mechanically scrub the media to remove the adhered contaminant.
Scrubbing may be achieved by first fluidizing the media bed by circulating fluid across the media to create a media slurry, and then circulating the slurry from the filtration vessel through a pump, typically a centrifugal pump. The shearing action of the media passing through the pump impeller liberates the adhered contaminant from the media and transfers it into the liquid phase. To remove the liberated contaminants from the filter a portion of the liquid phase and hence also a portion of the contaminants is removed from the filtration vessel for disposal or other treatment, with the cleaned media being returned to the filtration vessel with the other portion of the liquid phase and hence also other portion of the liberated contaminants. After the scrubbing has continued for a suitable period, the pump is stopped and the media is allowed to settle to reform the media bed.
However, in this pump scrubbing arrangement there is a risk that the media slurry may become too concentrated due to the removal of too much liquid, which may result in the media slurry becoming non flowable or “solidifying” within the scrubbing circuit, requiring significant remedial attention. Such a risk therefore requires continual monitoring of fluid being delivered into and removed from the system. Also, filtration vessels using a pump scrubbing circuit must ensure that the vessel has a sufficient freeboard volume, i.e., a sufficient volume in addition to the media volume, to ensure that a dilute enough media slurry can be achieved and maintained so that it does not solidify when the portion of the fluid is removed from it. However, the requirement of an increased freeboard volume results in an overall increase in the size of the equipment, which in many applications is undesirable.
Furthermore, the level of shear and other forces established within the pump may result in significant attrition of the media, reducing its useful life-span.